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NADCA DIE CASTING CONGRESS & EXPOSITION, (2012), Transaction No. T12-071*This article is an abridged version of the original one.
**This paper was presented at the North American Die Casting Association Die Casting Congress and ispart of the North American Die Casting Association Congress Transactions. It should not be republishedwithout prior approval from the North American Die Casting Association

High Thermal Conductivity Steel and its Application to Die Casting Tools
K. Namiki, Masamichi. Kawano (Daido Steel Co., Ltd.)Thomas Schade (International Mold Steel Inc. Hebron, Kentucky)
ABSTRACT
It is required in die casting operations not only to produce high enough quality products but also to shorten
cycle time. Increased usage of light weight components produced by die casting, have made reduced cycle
time a critical issue for die casters. To reduce cycle time, the application of high thermal conductivity
materials to die casting tools is effective as well as reinforced internal cooling of molds. Although W alloys
are high thermal conductivity metals, the price is too high to increase their applications. On the other hand,
it has been widely known that low alloy type steels have higher thermal conductivity than the most popular
5%Cr-1%Mo type die casting mold steel H13. But, they are generally supplied as pre- hardened steel: thehighest hardness is usually 40 HRC. DHATM-THRMO is a high thermal conductivity steel and highhardness of 48HRC is available by quenching and tempering. In this paper, examples of the application todie casting tools are introduced as well as the fundamental features of this grade.
INTRODUCTION
The roles of die casting molds are two folds. One is to inject and cast molten alloys in the cavity of molds
to make required geometries. Another is to solidify molten alloys by removing the heat as heat exchanger.
Needless to say, the latter is more important to produce good quality products with fine grain structure
without internal defects in short cycle time. When automotive industries are considered, aluminum die casts
are increasingly used for various components such as engine block and transmission case to produce lightweight vehicles for improved fuel economy. Therefore, shorter cycle time is one of the key issues inautomotive industries for cost reduction. Furthermore, fine cast microstructure with less defects in shortertime solidification contributes to further cost reduction and application of die cast products to wider variety
automobile components.
To reduce cycle time by faster solidification of molten alloys various efforts have been strived to improvecooling efficiency. Among them are increased internal cooling by the increase in water lines and their proper
layout. The distance between the top of water line and the mold surface is becoming shorter, for example.
These efforts, however, have accompanied with early stage gross cracking from water lines, especially fromthe crossing of water lines and oxidized internal surface. Another solution is to apply high thermal
conductivity materials such as Cu alloys and W alloys. Although thermal conductivity of these metals aremuch larger than tool steels, they have difficulties in actual applications as die casting molds mainly due to© 2021 Daido Steel Co., Ltd.

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their high price and poorer mechanical properties than tool steels, resulting in limited applications.
Furthermore, high thermal conductivity steels are able to improve properties required in die casting molds,
heat checking and soldering resistance, due to easier diffusing heat out in operations. Heat checking isthermal fatigue phenomenon caused by the repetition of heating and cooling on mold surface. The highest
temperature decreases when high thermal conductivity molds are used, which results in improved heatchecking resistance. The decreased surface temperature in operation is also effective to prevent erosioncorrosion on the surface. From these points, application of high thermal conductivity steels will be measures
to prolong mold life.
THERMAL CONDUCTIVITY OF MATERIALS
Table 1 shows thermal conductivities of various materials. Compared with the most popular hot work diesteel H13 used for die casting tools, lower alloy tool steels such as PX5 and NAK80 mainly used for plastic
injection mold steels show larger thermal conductivity. This effect of alloying elements was already reported
almost 55 years ago1). Recently one of these low alloy tool steels, named HDS1, has applied to actual die
casting molds for automotive and motor cycle components2). The drawbacks of these low alloy steels werelow hardness lower than 40 HRC due to low carbon and poor harden ability mainly due to low Cr content.
Therefore, applicable tools were limited to small molds which thermal stress were not highly applied.
Non-ferrous materials which show much higher thermal conductivity are Cu-Be alloy and W alloy. The
former is mainly used for plastic injection mold and the latter is for small pins and inserts for die casting
systems. The most popular W alloy, Anviloy, shows almost three times higher thermal conductivity thanH13, but it has draw backs such as expensive and brittle due to powder sintered material.
Table 1 Thermal conductivity of metals and steels.
Classification

GradeMain chemistry (%)

Low alloyed
tool steel

PX5*
NAK80*HDS1*
H13Cu-Be
Anviloy

0.2C--2Cr-0.5Mo
0.1C-3Ni-1Al-1Cu
0.22C-1.5Mn-1.9Cr-1Mo
0.38C-1Si-5.3Cr-1.2Mo-1V
2Be-CuW-Mo-Fe-Ni

Hot work die steel
Cu alloy
W alloy

Hardness(HRC)3040
3840 - 50
3335-40

Thermal conductivity(W／MK)
42424127130
80-100* Brand name of Daido SteelFUNDAMENTAL PROPERTIES OF DHA-THERMO
Based on these backgrounds new high thermal conductivity hardened and tempered type hot work diesteel was developed targeting to obtain higher hardness and improved hardenability3). By testing the effectsof alloying elements: Si, Mn, Cr, Mo, and V, on several properties: thermal conductivity, hardenability,toughness, the new steel named DHA-THERMO was developed. The hardenability was improved byadding the optimum amount of Mn and controlling Cr content to maintain high thermal conductivity.

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3/8THERMAL CONDUCTIVITY
Thermal conductivity at room temperature and elevated temperature are shown in Figure 1. At roomtemperature, thermal conductivity of DHA-THERMO is as 1.6 times larger than that of H13. Although thedifference decreases with elevating test temperature, DHA-THERMO still maintains higher thermalconductivity than H13 at 500 ℃ which is the estimated surface temperature of the molds in general die
casting operation.
TEMPERING HARDNESSHardness change with tempering temperature is shown in Figure 2. DHA-THERMO shows lower
hardness than H13 when tempered at 300 to 600 ℃. At higher than 600 ℃, however, DHA-THERMO
maintains higher hardness than H13. These behavior are derived from the chemistry, especially low Crcontent. By selecting tempering temperature from 610 to 680 ℃, aimed hardness 40 to 47 HRC areavailable.
HARDENABILITY
Hardenability is evaluated by CCT (Continuous Cooling Transformation) diagram as shown in Figure 3.
DHA-THERMO shows slightly inferior hardenability to H13 in terms of Bainitic hardenability. Figure 4
indicates the hardness distribution in different thickness of 200 mm dia. round bar when vacuum gas
quenched with 4 Bar and tempered at 580 ℃. Hardness in the center part of 100 and 150 mm long bars
are slightly lower than that of 60 mm long.

Figure 1 Thermal conductivity vs. test

Figure 2 Hardness vs. tempering temperature.

temperature.

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Figure 3 CCT diagram.

Figure 4 Examples of hardness in round bars 4Bar
gas Quenched and tempered.

DIE CASTING TEST
By 135t small machine, actual die casting test was performed. Considering that the cooling of the biscuitthat finally solidify is the key issue in operation, DHA-THERMO and H13 were applied to sprue core andtemperature was measured. The applied position and testing conditions are shown in Figure 5 and Table 2,respectively. The temperature on the sprue core surface was measured and the microstructures of thebiscuit part were observed.
Table 2 Die casting test conditions.
Die cast machine
Molten Aluminum
Die cast productInjection rate
Cycle time

135t (Toshiba)
ADC12, 700˚C
122mm x 122mm x14mm
650 ± g0.2m/sec., 1.6m/sec.
Total
27 secSolidification9 sec
Lubricant spray 3.5 sec.

Figure 5 Application of DHA-THERMO in die castingtest.
The temperature distributions around biscuit parts measured by radiant thermograph are shown in Figure
6. Temperatures were measured after unclamping and just before lubricating. The sprue core made of
DHA-THERMO showed lower temperature than that of H13 by 120 ℃. As shown in Figure 7, optical
microstructure of the center surface of the biscuit which touched with DHA-THERMO sprue core wasmuch finer than that of H13. It is derived from the faster molten aluminum solidification due to faster heattransfer through high thermal conductivity sprue core.

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Movable moldCastBiscuit

DHA-THERMO

Sprue core

H13

Figure 6 Surface temperature of sprue core.

DHA-THERMO

H13

Figure 7 Optical micrographs taken from biscuit surface.
By using this die casting machine, heat check resistance was also tested. Figure 8 shows the heat checking
appeared as of 10,000 shots on the surface of movable molds made of DHA-THERMO and H13. On H13mold surface, heat checkings were observed remarkably at gate side and the center, especially. Comparedto this, DHA-THERMO mold showed milder cracks at the gate and almost none at the center. Thissuperiority of DHA-THERMO was derived from less thermal stress due to lower highest surface in theoperation by larger heat transfer in the operation.

Gate side

DHA-THERMO

H13

Figure 8 Heat checking observed on the surface of movable molds as of 10,000 shots.

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APPLICATION TO DIE CASTING TOOLSDHA-THERMO has been applied to a wide variety die casting tools such as small insert, distributor, corepins and so forth, especially to the tools which need faster cooling. Representative successful examples areshown in Table 3. The advantages of DHA-THERMO over H13 are classified to three: quality improvementin Al die cast products, decrease in cycle time, and cost reduction of tools. Among the examples in Table 3,
noteworthy items are shown in detail as follows.
Table 3 Examples of application.
Advantage

Tools

Applications

Quality
improvement
ofdie castproducts

Center pinMotor cycle

Conventional
GradeH13,Core pinMotor cycle

H13

Split insert

Automobile

H13

Distributor

Automobile

Distributor

Automobile

High Mo
type grade
H13

Sprue core

Automobile

H13

Squeezebush
Distributor

Automobile

High Mo
type grade
H13Core pin

Automobile

Reduction incycle time

Cost
reduction

Automobile

W alloy(Anvilloy)

Hardness
Coating
49HRC
Salt bath nitriding
47HRC
Salt bath nitriding
42HRC
Salt bath nitriding
45HRC
Multi- layer coating
47HRC
45HRC
45HRC,
Nitriding+shot peening
45HRC
Ferritic
Nitrocarburizing
50HRC

Results
Improved yield rate
(Shrinkage reduced)
Improved yield rate(Shrinkage reduced)
Improved yield rate
Surface temperature
decrease
Improved yield rate
Deeper chill layer
Decreased surface
temperature by 45-80˚C
Reduction in cycle time by
18%
Improved cooling
Reduction in cycle time

Same life as W alloy
Substitute for expensive WalloyQUALITY IMPROVEMENT OF DIE CAST PRODUCTS
DHA-THERMO was applied to the center pin. Cast component was motor cycle Al wheel. Compared withthe conventional pin made of H13 heat treated to 49 HRC and salt bath nitride, the yield ratio of castproducts remarkably increased by the decrease in shrinkage．Another example in die casting of motor cycle
components is core pins for cylinder cover mold. Conventional pins were H13, 47 HRC and salt bathnitrided. Hardness and nitriding process for DHA-THERMO were the same. By substituting for H13, yieldratio was dramatically improved by 50 %. The part of the Al cast product where internal quality was much
improved is shown in Figure 9.
DHA- THERMO was used as split insert for automobile components. H13 used to be applied with thehardness 42 HRC and salt bath nitrided. The surface temperature decreased by 30℃ and the leakage rateby shrinkage decreased from 5 % to 0 %. Microstructure near surface was fine.
The other example used as distributor showed the fine optical microstructure as same as shown in Figure
7. The chill layer of the biscuit by using DHA-THERMO was 80 % deeper than by H13.

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7/8Figure 9 A part of Al die cast product where internal soundness was improved by the application ofDHA-THERMO core pins.
REDUCTION IN CYCLE TIME
One case study showed remarkable decrease in the surface temperature of a distributor sized 90 mm dia.
x 60 mm. Current steel was H13 with the hardness 47 HRC. By applying DHA-THERMO, surface
temperature decrease much lower than H13 just after taking out products from the mold.
The second one is the application to sprue core sized 132 mm dia. DHA-THERMO reduced cycle time by
18 %. Furthermore, the drop of the products due to un-solidified biscuit was completely prevented.
COST REDUCTION
As shown in Table 1, the thermal conductivity of W alloy is much larger than tool steels. When rapid
cooling is required, W alloy is limitedly used because of its high price. If the steel base core pins run almostin the same way, it will be very cost efficient solution. DHA-THERMO was applied in trial to one of corepins which Anvilloy was generally used. Two pins were heat treated to 50 HRC and tested without coating:one run 16,500 shots and cracked, another endured to planned life 20,000 shots without trouble．The
average life of Anvilloy was 15,000 to 20,000 shots.
The core pin used to 16,500 shots was shown in Figure 10. It cracked from the bottom of the corner at thestage of different diameter. This part was soldered as well. These failure modes, cracking and soldering, arecommon for hot work die steels.
The DHA-THERMO pin worked as well as the Anvilloy pin and lasted to the same life. DHA-THERMO
proved to be capable of being an inexpensive substitute of W alloy. As for the toughness, W alloy is inferior
to hot work die steels.

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8/8Figure 10 An example of core pin and failed area.
OTHERBeside the application shown above, DHA-THERMO is used for a wide variety core pins produced bycore pin suppliers. Advantages of this grade over H13 are all of three factors: quality improvement ofproducts, shorter cycle time and cost reduction. Furthermore, it is reported that galling is suppressed due
to decreased maximum temperature in operation.
It should be noted here that when the diameter of the pin is too small to machine water line in the center,
DHA-THERMO doesn’t work as the same as the pins with water line in them.
ACKNOWLEDGMENTS
We’d like to acknowledge Mr. Masahiro Hashiguchi, President of PT. Meiwa Kogyo Indonesia, and Mr.
Brian Grubb, AISIN Automotive Casting, LLC., for permitting to insert the application data in this paper.
*DHA is a trademark or registered trademark of Daido Steel Co., Ltd.

REFERENCES
1) K. Bungardt, W. Spyra: Archiv fur das Eisenhuttenswesen, vol. 36, p 257 (1965)
2) M. Ayabe, K. Shibata, H. Koyama, K. Ozaki, M. Kawano and T. Yanagisawa: SAE Technical Paper Series,
07M-113 (2007)3) M. Kawano and K. Inoue: Materia Japan, vol.48, p32, (2009)© 2021 Daido Steel Co., Ltd.

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